The present invention relates to a rotational angle detector, and more particularly, to a rotational angle detector used to detect the rotational angle of a rotor in a motor.
Power steering systems and air conditioners installed in, for example, automobiles are often driven by brushless motors. A typical brushless motor includes a rotor, which is formed by a permanent magnet, and a stator, which is formed by coils that are energized by the rotor. Electromagnetic force, which is in accordance with the power supplied to the stator, acts on the rotor. This rotates a rotary shaft of the motor. The brushless motor normally includes a rotational angle detector that detects the rotational angle of the rotor. The power supplied to the stator is controlled in accordance with the rotational angle of the rotor that is detected by the rotational angle detector. This controls the rotational state of the rotary shaft in the motor.
Japanese Laid-Open Patent Publication No. 2002-310609 describes a rotational angle detector that detects the rotational angle of a rotor. FIG. 1 is a perspective view showing the rotational angle detector. As shown in FIG. 1, the rotational angle detector includes a rotary plate 51, a magnet 52, and a magnetic sensor 53. The rotary plate 51 is rotated integrally with a rotary shaft 50 of a motor. The magnet 52 is spaced apart by a predetermined distance from the circumferential surface of the rotary plate 51. The magnetic sensor 53 is arranged between the circumferential surface of the rotor and the magnet 52. The rotary plate 51, which has an outer diameter that gradually increases in the circumferential direction, is formed from a magnetic material. The rotary shaft 50 of the motor is also formed from the same magnetic material. The magnet 52 includes a surface facing toward the rotary plate 51 and a surface facing away from the rotary plate 51. A J-shaped yoke 54 is arranged on the surface of the magnet 52 that faces away from the rotary plate 51. The yoke 54 guides the magnetic force generated by the magnet 52 toward one end of the rotary shaft 50. The yoke 54, the rotary shaft 50, and the rotary plate 51 form a magnetic circuit. The magnetic circuit forms a magnetic field in the clearance between the magnet 52 and the circumferential surface of the rotary plate 51. The magnetic sensor 53 outputs a voltage signal that is in accordance with the magnetic field.
In the rotational angle detector, rotation of the motor rotary shaft 50 rotates the rotary plate 51. This varies the size of the clearance between the circumferential surface of the rotary plate 51 and the magnet 52 and changes the magnetic field acting on the magnetic sensor 53. In other words, rotation of the motor rotary shaft 50 varies the output signal of the magnetic sensor 53. The output signal of the magnetic sensor 53 varies linearly relative to changes in the rotational angle of the rotary shaft 50. Accordingly, the rotational angle of the rotary shaft 50, or rotor, may be calculated from the output signal of the magnetic sensor 53. This would allow for the rotational angle of the rotor to be detected with high accuracy.
In the rotational angle detector described in the above publication, a cross-section of the rotary plate 51 taken along a plane extending perpendicularly to the motor rotary shaft 50 has a noncircular shape. Thus, the rotary plate 51 is rotated in an eccentric state when the rotary shaft 50 rotates. This may vibrate the rotary plate 51 and lower the detection accuracy of the rotational angle of the rotor.